How Do Glaciers Form?
Glaciers form over continents by continuous snow accumulation. As snow piles up, it has to be retained without melting.
Next, air squeezes out and compacts through sublimation. Then, crystals of ice form by going from a solid to gas, without turning into a liquid.
Gradually, it refreezes into grains of ice similar to the size of sand. Because the snow pushes together, it forms glacial ice.
Overall, glaciers can grow hundreds of feet in height. This is because ice is piling up on top without ever melting.
Continental glaciers vs alpine glaciers
There are two types of glaciers:
- Continental glaciers
- Alpine glaciers
The Arctic is not a glacier. It’s just sea ice. Continental glaciers in Greenland and Antarctica store roughly 99% of the world’s ice. The remaining amount of ice is in alpine glaciers.
According to NSIDC, the Antarctic Ice Sheet holds a volume of roughly 80% of freshwater ice on Earth. Whereas the Greenland Ice Sheet stores about 19% occupying 4/5 of Greenland.
The remaining 1% exists in almost 200,000 alpine glaciers dispersed throughout the world.
|Glacier Type||Area||Volume||Percent (Volume)|
|Antarctic Ice Sheet||14,000,000 km2||30,000,000 km3||90.9%|
|Greenland Ice Sheet||1,700,000 km2||2,850,000 km3||8.6%|
|Alpine Glaciers||680,000 km2||170,000 km3||0.5%|
Glaciers chisel away the land like a rake
Glaciers are powerful agents of erosion. They’re just second to water which is the best at eroding land.
Like a rake scraping the dirt, glaciers move very slowly leaving a lasting impression. For example, they alter landscapes into arêtes, cirques, horns, drumlins and moraines.
During the last ice age, glaciers scraped away Canadian rock and carried glacial debris on a path to New York. In fact, Long Island is the glacial debris from the sediment dumping of a glacier terminal moraine.
It’s easy to recognize glacial sediment because it’s unsorted material (glacial till). Continental glaciers scrape away at the land like a rake and dump boulders, dirt and powder rock along its track.
Shaping Land By Moving Ice
Glaciers have three main components to them:
- First, the “zone of accumulation” is where snow falls and adds to the glacier.
- Next, the “zone of ablation” is where the glacier loses ice most likely due to melting.
- Finally, the “firn line” marks the division between the zone of accumulation and ablation.
Alpine glaciers accumulate snow at the top of mountains. Then, they flow downhill in a fluid manner (ductile flow). They move much quicker than continental glaciers.
Continents glaciers move horizontally through basal sliding. Basically, there’s enough water at the base of the glacier so it removes most of the friction.
On average, glaciers move just centimeters a day. If the glacier travels along a downwards slope, speed increases.
Glaciers are Erosional Behemoths
Glaciers can tear down mountains faster than any other geologic process. As glacial ice flows in a fluid manner, it erodes the surface in two primary ways:
- Ice plucking fills between cracks and rips sediments by freezing and thawing.
- Abrasion drags, scratches and grinds down material along the base of the glacier through friction.
Together, ice plucking and abrasion incorporate material into the glacier. The embedded material can pulverize rock into finer bits of sediment “rock flour”. When the glacier recedes, wind can blow the fine dust. As it spreads along the surface, this “loess” helps fertility for soils.
Both continental and alpine glaciers use these two erosion mechanisms. But the scale for “abrasion” and “ice plucking” in continental glaciers is massive.
Glaciers Sculpt the Landscape
Glaciers are the ultimate sculptors of the landscape. If you take a look around, they carve out features like drumlins, kettles and valleys.
Here is some of the common terminology you’ll hear for glaciers:
When glaciers erode a valley, they create U-shaped valleys. Whereas streams carve out V-shaped valleys. For rivers, it mainly has one point of contact. But glaciers have contact everywhere which gives it a bowl shape.
At top of glacial valley, it has a head-wall called “cirque”. Cirques are concave in shape and form near or at the bottom of mountains.
Arêtes are steep knife-edge ridges that form on opposing sides. Both sides have been worn down by erosional processes creating a narrow ridge.
When arêtes erode three or more sides, the sharp-edge peak that remains is a “horn”. For example, the Matterhorn in Switzerland is one of the most notable horns formed by glacial erosion.
Fjords are glacial valleys that formed from glaciers. Between two steep cliffs, the area flooded out at the base is a fjord.
When glaciers move through mountains, the hills that aren’t completely removed are “drumlins”. These thick, low oval mounds point in the direction of the ice flow.
Alpine glacier erode rocks and the unsorted glacial sediment is carried to the end of the glacier at a “terminal moraine”. The endpoint where the glacier dumps rock and sediment is the point of furthest advancement.
When glaciers dump sediment at the end of the glacier, it can choke the carrying capacity of streams. When glacial ice melts, it creates a braided streams with the flow of water winding back and forth.
If glaciers move into an oceanic environment, large chunks of glacier can tear off. This process of calving can form icebergs as the torn ice floats away into the water.
Kettles are depressions that form from dead ice. When glaciers recede, large blocks of ice are left behind. If outwash gets dumped around it, ice melts in the middle. This creates a depression lake which are kettles.
Most glaciers have breaks (or crevasses) at the top of then. These brittle parts in glaciers are deep cracks due to the stress of moving on rocky terrain beneath.